CHAPTER X 
WHEN DEVELOPMENT GOES WRONG 
Differentiation with development - Underdevelopment, or dwarfing . Over- 
development, or giants- Arrested development of a single character or 
part. Overdevelopment of a single part- Doubling of parts. Fusing of 
parts When unit characters get misplaced - Abnormal growths 
Differentiation with development. The greatest marvel of 
development is differentiation. That two nuclei not only from 
different cells but from different individuals should fuse, absorb 
food, and divide and subdivide into not hundreds but thousands 
of others, —all this is wonderful enough, particularly when we 
remember that without this union neither would be capable of 
dividing at all. 
After all, however, the marvel is that with development comes 
differentiation ; that is, that the result of growth is not a lump 
of formless matter. On the contrary, here a leg, there an arm 
“buds” out; here an ear and there an eye or a tooth appears ; 
here a lung forms to take in air and there a heart develops to 
pump over the body the stream of digested food that we call the 
blood, —and so on, bit by bit, the whole complicated structure 
of the body arises, each part in its proper place; and not only 
that, but in general an exceedingly striking resemblance to the 
particular parentage results, so that the being which develops 
from a fertilized ovum of the horse, for example, is not only 
another horse, instead of a cow or a pig, but it is a particular 
kind of horse, depending upon the special individuals from 
which he was born. 
1 There are a few exceptions to this statement, but they are concerned with 
parthenogenesis, which is not involved in the subject matter here under 
discussion. 
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